If you're using the code I provide to set the crop rect, there's no need to involve GDI+ at all. Stay with RC6/Cairo and use the CropSurface method to get a new surface at the required coords and save it to disk. Something like:

Code:
mo_SrfOrig.CropSurface(mt_SelRect.Left, _
                                    mt_SelRect.Top, _
                                    mt_SelRect.Right - mt_SelRect.Left, _
                                    mt_SelRect.Bottom - mt_SelRect.Top).WriteContentToJpgFile App.Path & "\cropped.jpg"